The invention relates to a display device comprising a liquid crystal material between a first substrate provided with row or selection electrodes and a second substrate provided with column or data electrodes, in which overlapping parts of the row and column electrodes define pixels, drive means for driving the column electrodes in conformity with an image to be displayed and drive means for driving the row electrodes which, in the operating state, sequentially supply groups of p row electrodes with mutually orthogonal signals. Such display devices are used in, for example, portable apparatus such as laptop computers, notebook computers and telephones.
Passive matrix displays of this type are generally known and are increasingly based on the STN effect (Super-Twisted Nematic) so as to be able to realize a high number of lines. An article by T. J. Scheffer and B. Clifton xe2x80x9cActive Addressing Method for High-Contrast Video-Rate STN Displaysxe2x80x9d, SID Digest 92, pp. 228-231 states how the phenomenon of frame response, which occurs in fast switching liquid crystal materials, is avoided by making use of Active Addressing. In this method, all rows are driven throughout the frame period with mutually orthogonal signals, for example, Walsh functions. The result is that each pixel is constantly excited by pulses (256 times per frame period in an STN-LCD of 240 rows) instead of once per frame period. In multiple row addressing, a (sub-)group of p rows is driven with mutually orthogonal signals. Since a set of orthogonal signals, such as Walsh functions, consists of a plurality of functions which is a power of 2, i.e. 2s, p is preferably chosen to be as equal as possible thereto, i.e. generally p=2s (or also p=2sxe2x88x921). The orthogonal row signals Fi(t) are preferably square wave-shaped and consist of the voltages +F and xe2x88x92F, while the row voltage is equal to zero outside the selection period. The elementary voltage pulses constituting the orthogonal signals are regularly spread across the frame period. Thus, the pixels are then excited 2s (or (2sxe2x88x921)) times per frame period with regular pauses, instead of once per frame period. Even for low values of p, such as p=4 (or 3) or p=8 (or 7), this appears to suppress the frame response just as well as driving all rows simultaneously, such as in Active Addressing, but much less electronic hardware is required for this purpose.
However, it appears that the realization of grey scales by means of this multiple row addressing mode causes quite some problems because the frequency contents of the voltage at a pixel strongly differs for different picture contents when using the conventional method such as binary division of frames or when using the split level method for the functions used. Since the dielectric constant of liquid crystalline material is frequency-dependent, this may cause the liquid crystalline material to react differently at different locations in, for example, a matrix display, dependent on the picture information. This leads to artefacts in the picture, notably to different forms of crosstalk.
It is, inter alia, an object of the present invention to provide a display device of the type described above, in which a minimal number of artefacts (crosstalk) occurs in the picture.
To this end, a display device according to the invention is characterized in that the drive means present mutually orthogonal signals to p row electrodes for realizing at most (2n+4) grey values (n greater than 1) during (n+1) consecutive frames of different lengths, with a non-binary division of the frame lengths.
It appears that with such a choice of the number of grey values and the number of frames of different lengths, the differences in frame length may be small (particularly with respect to the customary binary division). Moreover, the ample choice of the number of possible adjustments of the effective value of the voltage across the pixel appears to provide the possibility of choosing a number of grey values which are spaced apart substantially equidistantly.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.